Technical Field
The present invention relates to a field device and in particular to a field device configured to convert a detected signal of a sensor into a predetermined voltage and then to output the voltage.
Related Art
Field devices are field installation instruments in process control systems and various types thereof are employed. As one type of field devices, a voltage output type transmitter which is configured to be operated at a lower current and thus has a lower power consumption is known.
FIG. 4 is a block diagram showing an example of a related-art voltage output type transmitter T. An analog output signal of a sensor 1 is inputted to a measurement control unit 2 to be subjected to a digital calculation processing for normalizing the signal to a predetermined range of voltage (1-5V), which is standardized by international organizations, and then is inputted to a D/A converter 3 to be again converted into an analog signal of 1-5V.
Also, the measurement control unit 2 is constituted of an A/D converter 21 for converting the analog output signal of the sensor 1 into a digital signal, a CPU 22 for generally controlling operations of the measurement control unit 2, and a modem 23 for communicating a signal with the external device.
The analog signal of 1-5V converted and outputted from the D/A converter 3 is outputted as an output signal to the external device through an amplifier 4 and a noise filter 5. Also, the noise filter 5 is configured to obtain a desired noise elimination characteristic by combining an inductance, a capacitor, a resistor and the like, but only a resistor is shown in the figure.
A linear regulated power supply 6 stabilizes an output voltage of a DC power supply 7 to a predetermined driving voltage depending on each part and then supplies the voltage to each part. A voltmeter 8 measures and monitors an output voltage of the amplifier 4.
According to the configuration of FIG. 4, it is not necessary to output a current signal of 4-20 mA, because the voltage is outputted. Accordingly, the transmitter T is operated only at a current of about 3 mA required in itself, thereby achieving a lower power consumption.
In Patent Document 1, a configuration of a three-wire low power transmitter is described.
[Patent Document 1] U.S. Pat. No. 5,245,333
As described above, the related-art low′ power transmitter T shown in FIG. 4 employs the general linear regulated power supply 6 as a power supply for supplying a driving voltage to each internal part of the device, and thus is configured to consume a constant current regardless whether a power supply voltage supplied to a primary side thereof is high or low.
Therefore, the current of about 3 mA required in the transmitter can be stably supplied to a secondary side, but because the constant current is always consumed, efficiency in terms of power consumption becomes poor if a primary-side voltage is high.
In order to prevent this, mounting a switching power supply instead of the linear regulated power supply 6 could be conceived.
Herein, because an operation current of the linear regulated power supply is determined by a secondary-side output current, the operation current is not changed even if the primary-side voltage is varied and thus an input impedance considered on the primary side is high.
On the other hand, the switching power supply is operated so that an operation current I thereof is decreased when the primary-side power supply voltage is high and increased when the primary-side power supply voltage is low. Namely, because the operation current I is changed depending on the power supply voltage, it seems that the input impedance considered on the primary side is lower as compared to the linear regulated power supply.
Also, the switching power supply is configured to transmit energy by switching the input voltage, but in order to prevent the input voltage from being significantly varied due to such switching operations, the input impedance thereof has to be decreased by incorporating a capacitor having a large capacitance or the like.
In this way, when the linear regulated power supply is substituted with the switching power supply, the input impedance of the primary-side power supply part is decreased. In addition, because a signal output of the transmitter having an output of 1-5V has a low impedance, it can be considered that a noise N from the exterior is intruded into the internal of the device to cause misoperation thereof.
In order to prevent this, for example, a filter or the like is arranged on the output part for the purpose of eliminating the noise.
However, in the low power transmitter T shown in FIG. 4, a ground wiring G are shared for the power supply and output voltages. As a result, an error voltage caused by a resistance component of the noise filter 5 provided on the grounding wiring G and the operation current I flowing through the resistance component is generated, thereby decreasing accuracy of the output voltage.
Also, in terms of the amplifier 4, an output of the amplifier 4 employs a common potential point as a reference. Therefore, if the error voltage exists in the ground wiring G, the reference for the output voltage becomes wrong, and as a result, a voltage measured by the voltmeter 8 includes an error.